Open deltoid muscle biopsy specimens from patients with idiopathic adult dermatomyositis, paraneoplastic dermatomyositis, childhood dermatomyositis, and idiopathic polymyositis, and from control patients were studied. Qualitative and morphometric capillary analysis by phase and electron microscopy was carried out. In the morphologic analysis the most striking difference was the presence of capillary damage and a higher capillary depletion in dermatomyositis as well as a higher capillary density in polymyositis. By electron microscopy, capillaries from patients with dermatomyositis showed mainly microtubuloreticular structures, loss of endothelial plasma membranes, and the appearance of abnormal cytoplasmic organelles. In contrast, capillaries from patients with polymyositis exhibited only minimal changes. By morphometric analysis, muscle capillaries in dermatomyositis had a significantly higher mean endothelial thickness than those in polymyositis. Finally, a significant topographic association between capillary damage and muscle fiber changes was observed only in patients with dermatomyositis. On the other hand, paraneoplastic dermatomyositis showed fewer structural and morphometric capillary changes than the other forms of dermatomyositis. We conclude that dermatomyositis is characterized by microvascular alterations that are absent in polymyositis. The topographic proximity of capillary changes to muscle fiber injury suggests that capillary damage may play a role in the pathogenesis of the muscle lesions observed in patients with dermatomyositis.